This invention relates to coaxial transmission lines and, more particularly, to a set of transmission lines having conductors which are of rectangular cross-sections crossing over each other within the confines of a planar plate from which the transmission lines are milled out.
Cross-reference is hereby made to three copending applications pertaining to microwave systems assigned to the same assignee; "Square Conductor Coaxial Coupler" invented by T. Hudspeth, R. V. Basil and H. H. Keeling, Ser. No. 468,826, filed on Feb. 23, 1983; "Ferrite Modulator Assembly For Beacon Tracking System" invented by T. Hudspeth, H. S. Rosen and F. Steinberg, Ser. No. 469,870, filed on Feb. 25, 1983; and "Coaxial Line To Waveguide Adapter" invented by T. Hudspeth and H. H. Keeling, Ser. No. 468,825, filed on Feb. 23, 1983. These applications are hereby incorporated by reference in their entirety.
Coaxial transmission lines are utilized for the transmission of microwave energy. The lines are particularly useful in that they support a TEM (Transverse Electromagnetic) wave over a wide bandwidth. A particular use of transmission lines, in general, is found in the construction of satellites which orbit the earth to provide communications between stations on the earth's surface. Such satellites carry antennas along with receiving and transmitting equipment coupled to the antennas for the relaying of messages transmitted between the earth stations via the satellite.
In order to insure that the satellite antennas are accurately pointed toward the earth stations for the receiving and transmitting of the signals between the stations, an antenna connects with a monopulse feed structure which provides error signals in two coordinates, azimuth and elevation. These error signals are utilized by control circuitry to accurately orient the antenna in a desired direction. The antenna may be physically moved by such control circuitry or, alternatively, in the case of a phased-array antenna, may be electronically steered by the application of phase-shift commands to the phase shifters of the antenna system. In the case of radar systems which also employ a monopulse feed, the control circuitry has utilized both coaxial cable and waveguide for the transmission and combining of the microwave signals of the antenna for the development and processing of the azimuth and elevation drive signals.
However, in the case of a satellite, it is essential to fabricate the microwave circuits in a format that insures a high degree of reliability, and also provides for a relatively small physical size and weight for installation in the satellite. A particular form of microwave circuit structure that is useful in the construction of satellites has the form of a planar plate of a soft, light weight, electrically-conducting metal such as aluminum. The aluminum is readily machined to provide channels which serve as the microwave transmission lines.
With respect to the fabrication of coaxial transmission lines, it is noted that the channels are of a rectangular, preferably square, cross section, the walls thereof serving as the outer walls of a coaxial transmission line. The inner conductor of the coaxial transmission line is fabricated of the same metal, preferably, and is formed with a corresponding rectangular cross-section. The resulting structure is, thus, a square coaxial transmission line. The transmission line is completed by the placing of a cover plate above the base plate in which the channels have been machined, the cover forming the fourth wall of the outer conductor of the square coaxial transmission line.
Such a transmission line can be accurately fabricated in that the outer conductors are formed by a milling operation and that all critical dimensions of the inner conductor can also be attained by a milling operation. Suitable dielectric spacers positioned between the inner and outer conductor support the inner conductor at its proper location relative to the outer conductor of the transmission line.
A problem arises in such a mode of construction in that, in the case of a complex microwave circuit, such as a circuit including hybrid couplers, power dividers and combiners, it may be necessary for one transmission line to cross over a second transmission line without any coupling of the microwave energy between the two lines. Such a crossover greatly facilitates the interconnection of the various components of the circuit since, without such a crossover, it may be necessary to reroute the transmission lines and to rearrange components of the circuit so that all the components and all the transmission lines can be accommodated within the structure of the planar plate. However, no such transmission line crossing has been available heretofore.